Cyrstal oscillator circuits



i A7 2,444,349 Y 2 Sheets-Sheet l PHASE .SH/F TER "1 FEEDBACK AND 0.6. AME

LINEAR POWER MPL/FIL' C. W. HARRISON CRYSTAL OSCILLATOR CIRCUITS LINEAR POWER RECT/F/ER By C W HARR/SON FIG.

June 29, 194s.

Filed Oct. 3l, 1945 PIER) L/N/TlNG AMPLI June 29, 1948. C, W` HARRSON 2,444,349

CRYSTAL OSC ILLATOR CIRCUITS Filed Oct. 3l, 1945 2 Sheets-Sheet 2 /Nl/ENTOR y C. W HARR/SON A TTORNEY Patented June 29, 1948 'CRYSTAL osciLLA'roa omcUi'rs Charles W. Harrison, Flushing, N. Y., assignor to Bell Telephone Laboratories,y Incorporated, New York, N. Y., a corporation of New York Claims.

This invention relates to oscillation generators and particularly to constant amplitude crystalcontrolled oscillation generator circuits. This application is a continuation in part of my copending application for Test circuit, Serial No. 576,659, nied'lebruary 7, 1945.

AOne-cf 'the objects of this invention is to improve the'frequency and amplitude stability of oscillation generators.

Another object of thisinvention is to provide a-tuningarrangement capable of selecting fine increments of frequency.

lAnother object of this invention is to proa vide av stable oscillator capable of utilizing high degrees 4of automatic amplitude control.

In accordance with this invention, a stable oscillator circuit may be provided which is adapted to overcome diiiiculties with motorboating, and which is capable of oscillating with even rather poor quality piezoelectriccrystals, """`"antl"cap`ble'"cfin'gmadiisover'a wide voltage range to suit a great variety of different `crystal elements that may be used in circuit therewith; and which is capable oi maintaining its output voltage at-a very constant amplitude level, and-which may be provided with a tuning arrangement capable of selecting fine-increments of frequency.

'Oscillators in accordance with this invention may comprise an electronic amplifier, which may be eitherA a limiting amplifier alone or a limiting amplifier followed by a linear amplifier, the amplier apparatus being followed by a tuned stepdown transformer connected across the output circuitofl the amplifier, and a resonant crystal circuit-comprising a series-connected piezoelec- .tric` element and capacitors connected across the secondary winding of the tuned transformer, anda feedback path extending from a point in the resonant'crysta-lcircuit to the vinput circuit of 'the limiting amplifier. In cases where the "limiting amplifier and a separate linear amplifier are'utilized, an automatic volume control circuit including a-rectier, a4 direct current amplier and gain control means may be provided and designedwith` suflicient capacity to provide automatic amplitude control for the linear emplifler. The'automatic amplitude control circuit may be arranged tohave a 'time constant which is differentl from and somewhat slower than that of theA limiting' amplifier, in order to secure good youtput voltage stability free; from motor-boating." 'By rriotor-boating is meant a low frel quency amplitude -'disturbancewhich is common .to most forms of automatic 'amplitude-controlled vApplication October 31, 1945,S.erial No..625,896

(Cl. Z50-36) oscillators. 1 If desired, suitableshielding maybe provided around thevpiezoelectric crystal body and also around the capacitors of the resonant crystal circuit, as disclosed in my parent application, Serial No. 576,659, hereinbefore referred to.

Osciliators in accordance with this invention may be utilized to develop a substantially constant voltage .across theelectrode terminals of the piezoelectric crystal element, independently of the quality of the crystal element over a quality range normally considered as vusable in quartz crystal plateaand may be used as a .basis for testing and for specifying theA maximum voltage rating Aon various typesof .quartz crystal plates, asin the testing of vsuch crystals during manufacture. To ,help in protecting such crystal elementsagainst damage from an excessive voltage being accidentallyapplied across the terminals of the .crystal element, it is useful to know the rating of the maximum surge voltage that the particular crystal element can withstand since the use in practice of a crystal element at voltages in excessof its maximum rated value or in excess of its proper working. voltage often results in. failure, due to. internal heating andl fracture of the crystal dielectric. Accordingly, it is often desirable thatI the manufacturerbe able to supply piezoelectric crystals with a rating for the maximum surge voltage that the crystal elementV can be expected to safely withstand.

For a clearer understanding of the nature of this .invention and the additional advantages, features and objects thereof, reference is made to the .following description taken inconnection with the accompanying drawing in .which like reference characters represent like or ysimilar parts and in which:

Fig. 1 is a diagram illustrating, in block diagram form, a constant-voltage, crystal-controlled, two-stage oscillation generator circuit provided with automatic amplitude control for the linear amplifier stage thereof;

Fig. 2 is a diagram illustrating, in schematic form, an oscillation generator circuit of the genera-l typeshown in Fig.. 1;

Fig. 3 is a diagram illustrating a modication of the oscillation generator circuit shown in Fig. 2; and

Fig. iis a diagram illustrating .a modifi-cation employing a single vstage amplifier.

'Referring to thedrawing, Fig. 1 isa circuit diagram illustrating atwo-stage, constant-voltage and constant-frequency oscillator, the frequency of whichI is controlled mainly by a suitable piezoelectric crystal element l having any suitable electrodes 2 and 3, and which is adapted to supply a substantially constant amplitude of output voltage eo, at the output terminals 4 and 5 thereof. As illustrated generally by the block diagram labeled 'l in Fig. 1, the first amplifier stage 1 of the oscillator may comprise an amplier 1 of the limiter type, the output of which is fed by suitable coupling means to the input of the second amplifier stage 8, which is illustrated generally by the block diagram 8 in Fig. 1. The second amplifier stage 8 may comprise a power amplifier of the linear type, the output of which is fed to the input circuit of a tuned transformer T having a primary winding P shunted by an adjustable tuning condenser Ct, and a secondary winding S which is shunted by a resonant crystal circuit comprising the series-connected piezoelectric crystal body I and condensers Cs and Ck. A feedback circuit ID may supply output voltage en, from across the capacitor Ck to the input circuit of the limiting amplifier 1. A suitable switch I2 may be provided in an automatic amplitude control circuit I4 in order to supply voltage from either of the terminals 2 and 3 of the crystal element I to an automatic volume control system I5 which may comprise a rectifier and a direct current amplifier, the output of which is fed over the circuit I4 to the input circuit of the linear power amplifier stage 8.

The frequency of the oscillator illustrated in Fig. l is controlled mainly by the piezoelectric crystal element I, through feedback from the capacitanoes of condensers Cs and Ck. The automatic volume control circuit I4 is provided in order that the amplitude of the voltage ei generated across the secondary winding S of the tuned transformer T may be held essentially constant at all times and at all frequencies. The circuit is constructed with component elements of values suitable for oscillation at the desired frequency, and an adjustment for insuring operation at the desired frequency is provided by the phase shifting network comprising the tuned transformer T having its -primary winding P shunted by the variable tuning condenser Ct. By an adjustment of the tuning condenser Ct, the frequency of the circuit may be shifted the slight amount that may be necessary to obtain a suitable resonance corresponding to the resonant frequency of the crystal element I. The ouput voltage eo appearing across the condenser Ck may be fed back and applied to the input circuit of the limiting amplifier 'I.

The oscillator circuit illustrated in Fig. 1 is capable of oscillating when using any crystal body I that is usable in other types of oscillator circuits and, moreover, is capable of permitting high degrees of automatic volume control from the circuit I4 in order to maintain the generated voltage constant while adjusting the crystal circuit frequency for resonance operation. When the generated voltage is constant, resonance of the crystal circuit is essentially indicated by maximum crystal current, and oscillation is maintained at that resonant frequency. The adjustment to obtain maximum crystal current is such that the phase shift through the a oscillator loop is 21m. where n=0, 1, 2, 3, etc., the phase shift and resulting frequency of oscillation being varied by adjustment of the tuning condenser Ct associated with the tuned transformer T, and the magnitude of the generated voltage being held constant bythe automatic amplitude control system I5 which may be similar to automatic volume control systems applied to amplifiers in radio systems. Manual control of the magnitude of the generated voltage ei may be provided by an adjustment of the bias Voltage of the automatic amplitude control system I5 and in this way the maximum or start gain may be made independent of the setting of the amplitude control system I5.

The voltage e1 may be, in some cases, generated by a single turn S of wire forming the secondary winding S of the transformer T, and the output voltage eo may be used as the feedback voltage supplied over the feedback circuit I0 to the input circuit of the limiter amplifier 1. The value of the voltage ratio of eu/ei represents a voltage gain. This voltage gain is of course a fraction of the relative values of Cs and Ck. This type of oscillator circuit may present a pure capacity across the crystal element I for all frequencies equal to the value of CkCs CIc-i-Cs Also, it may be operated over a wider reactance versus frequency range than earlier oscillators, with the amplitude of oscillation arbitrarily selected and held constant at that selected level and with high degrees of automatic amplitude control adjustable over a wide range.

While the limiter amplifier 1 and the linear amplifier 8 may be combined in a single tube amplifier as disclosed more fully hereinafter in connection with Fig. 4, the two are made separate when used with the automatic amplitude control rectifier and direct current amplier I5, as illustrated in Figs. 1, 2 and 3. The application of the automatic volume control rectifier circuit I4 to this type of oscillator makes it desirable to separate the linear and limiter amplifiers 'I and 8 and to control the linear amplifier 8 slowly enough to permit the limiting amplifier I to absorb the changes in gain, thus damping out the critical transient caused by the automatic volume control rectifier I5, which maintains the output voltage constant with variable input to the power amplier 8. In order to prevent motor-boating, the time constant of the limiter amplifier 1 may accordingly be made faster than that of the automatic volume control system I5 especially for high degrees of automatic volume control provided by the circuit I4. It will be noted that such arrangement provides a stable amplitude control for the oscillator, with a tuning arrangement capable of selecting fine increments of frequency for the crystal circuit. A suitable low capacity diode type vacuum tube voltmeter (not shown) may, if desired, be used to indicate the voltage ep across the crystal element I. Since ep is greater than e1, the voltage across the crystal element I may be measured from the connection between the crystal element I and capacity Cs, and ground. The power supply system used may consist of any suitable supply voltage source or sources, which may be transformer sources adequately filtered and of conventional design.

Fig. 2 is a circuit diagram illustrating in schematic form an embodiment for the two-stage oscillation generator illustrated in block diagram form in Fig. 1. As illustrated in Fig. 2, the limiting amplifier 'I may comprise a suitable overloaded vacuum tube VT-I giving a relatively at response characteristic with a plate load of relatively low impedance, and the linear power ampller 8 may comprise a single vacuum tube VT-2 or VF--lr or lmayfccmiprise two 'paraiieleeon- -neeted^=- vaeuinn tubes -vT-lz @and VTi-3, such -as typefAiG'I vaeuum-powertubes, the outputoi' which is fed into the-antiresonant primary lWinding circuit P'Yofthetuned step-down transformer T,` the secondary-windings 4of which 'provides a voltage-f e1 which maybe of f row magnitude-1 as compared to fthe voltageesaoross the crystal electrodes t' and 3.

-The iinii'tingliamplier'- Fruity-be either-cf L the overloadedv variety -or -one whose out-put Vis limited by an automatic volumeeontrolcircuit 'that-operatesonlyon that stage. "In Fig. 2, the limiting ampliner tube V-I'Iisvsho'cfr'rl as an' overloaded vacuum:` tabel'stage'' 'that drives thelinearamplieraiat-aconstant lpeak -level independently oi -themagrii-tude and vvariations of -the out-put voltage Aec `over' the range' in-which itis to be used. The linear anrplier 8'consisting-of the two lparallel-'connected vacuum tubest VfB-2 and 'VT-3 has its' tuned plate -circuit-'inductively coupled by the transformer T to the'ci'ystal-circuit. The tran'sformer'' may be a stepd'own transformer T ofthe over-coupled 'un'it' type-wound, forexanrp'le, -on an `iron dusty toroidal' wco'il` form. The secondaryWinding S'of the transformer T may cons'ist'o'f one or two turns of wire in a particular example, and may act as' a; generator tothe crystal circuit. The crystalcircuit may consistA of a suit- `able* piezoelectric crystal element |-connected in series circuit relation with two series-'connected capacitors CsandCk. 'The capacitor Cs maybe variable, and the capacitor'Ck may be fixed and th'ef capacitance of the condenser "Cl'c' mayv be greater than that of the condenser Cs. The'voltage eli-'developed across the condenser Ck or Co, if `used, may be fed back and used' to excite the grid circuit 22 of the vacuum tube VT-'Iv of the limiting amplifier 'l The limiting amplier' Il lii of the overloaded varietyimay comprise a'vacuum tube VT--l hav- .ing conventional electrodes Which-may, as illustrated -in Fig. '2. comprise agrounded cathode 2E? heated' by a suitable cathode heater 2l ener- -g'zed by a suitable power supply source (not shown), a control grid electrode 22 'connected 'through a suitable "grid resistorRl with the grfounded'cathode'zt, a 'screen grid electrode 23 "connected through -a sitabiescreengrid resister' R2 with the positive 'terminal `of'a suitablepower supply-'source BL'a suppressor grid' elect-rodean connected'with' the :grounded cathode '28, and' a plate electrode-l or anode 25 connected through a suitableplate resistor'f'RS withtlie-positive (-le) lterminalfoi lthe plate 'supt nectedftbroughsuitablerestors R7` and connected through theresistors Rlwith' the positive terminal of suitable power "supply source. B2, suppressor' grid 1 electrodes 2li-oonneotcd with .the cathodes 20;fand.plate-electrodes 2 5 interconnectedr .throught irritable:` resistors R6 and connected withV a suitable power isuppiy source B2 through the resistors R6 and R8. l"mie condensers' C4 may-be conventionalfby-passcon densers. The outputs from the anodesioi the linear.' power vamplifier tubes i VT-2: and 1 VIL-13 are connected across the'input or primarywinding' Pof the tuned transformer T.

As 'illustrated inFig. 2y the. transformer T fhas its-primary winding P connected-with thefoutput circuits ofthe linear amplifier `tubes VT-2'-and VT-Sthrough the plate resistors R5, and thc condenser C6. The secondary Winding Soi the transformer Tis connected in .circuitiwith the crystalelement l, and may,"if desired.' be shunted byfa resistor R9. The transformer T may be a step-down transformer providing a relatively' small voltage e1 across'. the secondary windings' thereof. The output voltage-ei generated across' the terminals-of 'the secondary winding S of the transformer T-is Vmaintained constant by'meansof the automatic volume control circuit Mand isapplied to the series resonant circuity comprising the lpiezoelectric crystal body I 'and the series-connected capacitors Cs, -Ck and Co. The condenser Co 4may bei'omitted-if desired. VThe transformer T has its `primary Winding P shunted by the tuning condenser Ct andis thereby adapted tofadjust the resonance of the secondary `Winding S thereof and. to provide a low impedance source of voltageei-across the secondary-winding S of the tuned transformer T. Accordingly, the tuned transformerT functions as a phase shifter in the output circuit of the linear amplifier tubes VT-Zcand VT-f3'and maybe utilized tio-adjust the resonant frequency of the oscillator by adjustment of the tuning condenser Ct. In cdditionVthe Vtuned-transformer T functions as electric waveiilter for filterinsf out undesired harmonics in the circuit.

The oscillator tubes .VT-4,' VT--Z` andVT-S providecurrent to theoutput circuit 4. through the transformerT and the oscillator is provided with automatic amplitude control. comprising the automatic. volumev control rectier and amplifier I5. The frequency of oscillation liswundercontrol of voltage derived from the crystal circuit comprising the piezoelectric crystalbodyl connected in series circuit` relation with the seriesconnected capacitors Cs and Ck and the secondary -Winding S of the transformer T. The capacitors Cs and Ck divide the voltage in the series resonant crystal circuit and supply a voltage-Which may be fed back over the feedback circuit l to the input circuit of the limiter amplier tube VT-L in proper phase4 to: maintain oscillations at the effective oscillating frequency. the frequency of oscillations being controlled mainly by the frequency of the crystal element I, and the output voltage eo being under-automatic amplitude control.

The tuning capacitor Ct may be-adjusted vfor minimum impedance of the` plate. load -ofthe linear amplifier tubes VT-Z and VT-3. in order toenable an incremental adjustment ofthe frequency ofoscillation. The minimum impedance grids 22 of the linear amplifier tubes VT-z-and the secondary winding S of the transformer T comprises the piezoelectric crystal element I connected in series circuit relation with the capacitors Cs and Ck which are adapted to be adjusted to operate at the resonant frequency of crystal and capacitor circuit, and used as a means for developing an output voltage eo across the low impedance of the condenser Ck, to be supplied over the feedback circuit I Il to the input circuit of the limiting amplier 1. The automatic amplitude control circuit I 4 includes a rectier the output of which is used as a control bias on the control grids 22 of the linear amplifier tubes VT-2 and VT--3. High degrees of automatic voltage control are possible with stability using this arrangement where the time constant of the limiting amplifier 1 is different from and preferably faster than the time constant of the automatic amplitude control circuit I 4. By utilizing a direct current amplier in the output of the rectifier I5 and by controlling more than one oscillator tube such as the tubes VT-2 and VT-3, very high degrees cf automatic volume control may be realized, with stability of operation. By means of the switch I2, the voltage on either side 2 or 3 of the crystal body I may be held constant.

Since the output voltage eo is developed across the relatively low impedance of the condenser Ck, and has been filtered twice, once by the tuned primary circuit P and Ct of the transformer T and second, by the resonant crystal circuit itself, the resulting output voltage eo is substantially free from electrical harmonics. The effective capacity across the crystal element I may be the same as the value of capacity in series with it when the condenser Ct is tuned for resonance. The capacity Ck forms a part of the series capacity in the resonant crystal circuit and may also be used as a, means for adjusting the frequency of oscillation to exact resonance of the crystal circuit. The amplitude may be varied for a given crystal element I by adjusting the ratio of the capacitance of the condenser Cs with respect to the capacitance of the condenser Ck. As an example, if the voltage appearing across the crystal element I is needed to be held to a low value and still insure maximum sensitivity to start the oscillation thereof, the feedback voltage eo could be made to equal a greater portion of the sum of the voltages es and en across the condensers Cs and Ck. In this way, the voltage across the crystal element I may be controlled at any frequency between the resonant and the antiresonant frequencies of the crystal element I at levels of voltage which may be, for example, less than 1.0 volt and at the same time stability and assurance of starting oscillations is obtained.

The crystal element I possesses two resonant frequencies, namely, a series resonant frequency which is determined by the effective inductance and the effective capacity thereof, and an antiresonant frequency which is determined by these quantities, plus the paralleling capacity which comprises the static capacity between the electrodes 2 and 3 of the crystal element I and any capacity connected thereto by the crystal holder and lead Wires within the crystal holder. At frequencies other than the series resonant frequency, the paralleling capacity together with the associated shunt loss of the holder enters into determining the performance of the crystal element.

The piezoelectric crystal body I may resonate between its resonant and its antiresonant frequencies, and the impedance of the crystal element I operating within this frequency range appears as a positive impedance or in effect as an inductive reactance in series with a resistance. Any oscillator that operates the crystal element I in the positive region of the reactance versus frequency characteristic thereof exhibits capacity reactance and negative resistance paralleled across the terminals 2 and 3 to which the crystal element I is connected. The operation of the crystal element I when connected into the oscillator circuit will be influenced by the magnitudes of these two terms, and the combination will operate at such a frequency that the total reactance is zero and at such an amplitude that the total resistance is zero. The performance of the crystal element I will therefore not depend solely upon its own characteristics but will involve also the impedance of the remainder of the oscillator circuit associated therewith, The circuit of the oscillator provides an effective capacity which is composed of grid and lead wire capacities and capacity introduced from the plate circuit. The frequency at which this combination exhibits antiresonance is the oscillating frequency. Accordingly, oscillation of the crystal body I may take place at any frequency between its resonance and antiresonance frequencies with the appropriate selection for the capacitance values of the series condenser Cs or Ck. Also, this type of circuit will permit oscillation over a wide range of voltage operating amplitudes of the crystal body I as determined by the relation eX- isting between capacita-nce values of the condensesr Cs and Ck. Thus, if the capacitance value of the condenser Ck is small and that of the condenser Cs is large, the amplitude of voltage across the crystal element I may be relatively low.

As hereinbefore mentioned and as shown in Fig. 2, automatic amplitude control, commonly referred to as automatic volume control or AVC, is applied to the linear power amplifier 8 comprising the vacuum tubes VT--Z and VT-3, which are separated from the limiter amplifier 1 comprising the vacuum tube VT-I In order to apply a high degree of automatic amplitude control to the oscillator, the input voltage from the feedback circuit I0 to the Vacuum tube VT-l of the limiter amplifier 'I may be made above the threshold of limiting by an amount exceeding the Variation in the loop caused by the automatic amplitude control circuit I4, thereby to enable the limliter tube VT-I to absorb the changes in the gain of linear amplifier tubes VT-2 and VT3, such that ir=1 at all times. In order to permit damping of transients set up by the changes in gain occurring from the action of the automatic amplitude control circuit I 4, the time constant of the limiter amplifier 'I including the tube VT-I may be made fast compared to that of the automatic amplitude control circuit I4. The input of the linear amplifier including the vacuum tubes VT-Z and VT-3 is held constant by the action of the limiter amplifier 'I including the tube VT-I, and gain changes in the linear power tubes VI2 and VT-3 due to variation of the capacitance of the tuning condenser Ct are absorbed by the automatic amplitude control circuit III, while the variation of activity of the crys'- tal element I is absorbed by the limiting amplifier tube VT-I.

'I'he automatic amplitude control system I5 may comprise any suitable AVC rectifier and direct current amplifier. As illustrated in Fig. 2,

au sito the automatic amplitude control-'circuit I loom"-l prises conventional electron tubes T4 andTl Thetube T may comprise of a. simple 'diode having a plate 32, a suitable cathode 36, and a lament 3.1, and which is coupled to thecrystaicinl cuit through a capacitor CII). 'IheztubezTSeis providedA with a. load resistor RII, a. radio ircL4 quency grounded cathode 36 grounded through a capacitor CI 1, and a radio frequency filterlcom prising a resistor RI2 and a capacitor;CI I,--the arrangement being such that thevoltagezappearingacross the. capacitor .CII will loe-pure directeurrent. The second tube T4 has a cathode 20; a suitable cathode heater 2l, a grid; electrode 30,19. plate electrode 3l connected with the.: positiveA terminal of a suitablepowersupply -source B3,I a. grid voltage control resistor Rlvtoi deter-l mine the range of the level control,y and :a'pIato electrode 33 connected with'theoutput circuit :I4 leading to the input of the linear power amplier 8-ofthe oscillator. Theplate electrode-33 oithe tube-T4- is connected through a suitable-resistor Rill to the initial bias potential for thefivacuum tubes VT-2 and VT-3. The plate electricidad! of thetube T5 is connected through a suitable-'ree si-stor RII to the cathodeelectrode-r 36; and through a. resistor RI2 and a condenser GII `to act asa lter. The cathode 20 of the tnbefl is connected. toI one side of the manual level com trol potentiometer resistor RI4 which is:connectfed in series with the load resistance.Ri3 the-re-` sistor,v RI3 being. connected. at .its other end .with the negative terminalofasuitableipower supply source B4.

The direct current amplifier portionzof thetube T4 of the automatic level .control system I5-'xapiplies a variable negative voltage to :thai-control grids 22 .of the vacuum tubes VT-2 and VT3 of the linear amplifier 8, and is controlled by nal voltage level of the circuit I4 which is rectiiier by the rectier T5,A the resultant direct current. voltage being applied to the grid electrode 30 of. the direct current amplifier T4.' As this latter voltage increases in value, the. cathode 201'0! the direct current ampliiler .T4 is driven negative with.respect to the chassis ground G. The elec-v trode plate 33 in the direct `current amplier tube Tk is then positive with respect tothe cathode l2U and draws current, causing-a negative voltage to appear across the load resistor RID.' This nega-A tive voltage is applied over the circuit I4 to the control grids 22 of the` linear amplifier tubes VT-Z and VT-3, thus reducing -the gain` of the linear amplier stage 8. The voltage level'point at; which vthe control operates is the level -at which the cathode 20 of the direct current ampliiler T4 becomes negative with respect to the plate electrode 33, and this point may be-'varied'by using the level control potentiometer R-Mtoapply an adjustable direct current voltage to-the control-grid 30 of the direct current'ampliiierT4- The grid electrode 30 of the tube T4 is connected to a resistance potentiometer R15 which deter# mines the range of` control possible by the level control R14.

Accordingly, in the automatic volume control system I5, the potential of the cathode 20"of. the ube T4 may be varied maneggi/lection by adlustrri"itci"13`ri' tap 35 'of the potentiometer R14l `in order to select the negative bias that.de termines the magnitude ofthe output voll-.agg e1 across the translormer secondary windings corrtrolle'dvby the automa-tic volume control circuit |Ii,-andv4 thus permit the hgain at: startingto be a maximum, the bias then being a minimum at the time before the automatic volume control `circuit I4 has any signal to operate on. Accordingly, this circuit may be expected to start and oscillate all piezo-electric crystals, capable of oscillation in other circuits. The crystal voltage ep may be almosta linear'function of the rotation of a straight line type of potentiometer RI4 used as the level control, which indicates that the direct current amplier T4 may work on a linear portion of its plate current-grid voltage characteristic.

As mentioned hereinbefore, this oscillator is adapted to operate at any frequency between the resonance and antiresonance frequencies of the crystal element l and is capable of maintaining the output voltage eo constant when the tuning adjustment of the condenser Ct varies the frequency for maximum voltage. Moreover, this oscillator permits the control of some 30 decibels or more of automatic volume control at the operating level of the crystal element I, and is free from amplitude disturbances for very high degrees of automatic volume control. When the time constantof the automatic volume control circuit I4 is made as fast Ias possible, greater stability of constant output may be obtained. Amplitude disturbances arise for the most part from the low frequency gain provided: through the automatic voluzme control circuit I4 which may provide an additional feedback loop and at some frequency the phase :may =be right to sustain an additional low frequency oscillation'and thus cause the oscillator to oscillate at'two frequencies Vsimultaneously.

It will be notedthatthehigh degree of automatic volume control may be accomplishedin this oscillator by the separation and independence given the limiter amplifier-1 .from the linear power amplifier 8,- wherein the limiting action provided bythe limiting tube VT--Il is above the threshold of limiting vbysan amount 'exceeding the variation inthe loop caused by the automatic volume control circuit I4' and the limiter 1 is thus enabled to-absorb the Achanges in the gain of the linear amplier 8. Moreover, the time constant of the limiter I maybe made fast compared to that of the automatic volume controlfcircut I4 to permit the'-da'inpingfl transients-set up by such changes in the gainthat occur 'as a. result of the 'operation of the automatic volume control circuit Fig.3 is a circuitdiagranrof an oscillation generator similar to that shown inFlg'. 2, except for the omission oifone of the two' vacuum tubes VT-Z and'. VT- *from the4 linear amplifier 8 shown in Fig-:2.: Accordingly, as showniin Fig. 3, the linear poweramplilier `incluzies a single vacuum tube VT-'2, and the circuit' in other respectshis'. similar tto .that shown schematically in Fig-2;

Asan villustrative' example inv a particular case, the resistance and capacitance andotherrvalues of the component elements of'an oscillator circuit as illustratedinFig; 3 of about 5 megacycle's per second :maylbe roughly as follows: resistor RI- about 470,000 ohms', resistorv R2 about 33,000' ohms, resistor R3 about 1,000'ohins, inductance L about 20'microhenries, resistor R4 about 500,000 ohms, resistor R5 about 270 ohms, resistor R' about V22 ohins,.resistor RT about 33,000'ohms,- resistor R0 about'500 ohins, condenser CI' about .001 microiarad, condenser-C2 about 50 micromicrofarads, condensers C3 and C4 about .001. :microfarad each,acondenser Ct about -to micromicrofarads or other values to suit the circuit tuning, condenser C6 about .001 microfarad, con- 'denser Cs about 0 to 100 micromicrofarads or Fig. 4 is a circuit diagram of a crystal con-` trolled oscillation generator using a single oscillator tube VT-I functioning as a limiting a'mplifier. The vacuum tube VT--I may be any sultable limiting type vacuum tube which provides some gain up to the region where the tube characteristic levels o. As illustrated in Fig. 4, the circuit comprises only one such oscillator tube VT-I. The crystal controlled oscillator in Fig. 4 comprises in addition to the limiting vacuum tube VT-I, the tunable transformer T, the crystal and condenser circuit in the secondary winding S of the tuned transformer T, and other co'mponent elements as hereinbefcre described. A condenser C disposed in the feedback circuit I may be used to couple the output and input circuits of the vacuum tube VI. The condensers CS and Ck may be used to divide the secondary circuit voltage and rprovide a reduced voltage in the feedback or circuit I0, and for other purposes as described hereinbefore.

Accordingly, the oscillator circuit as illustrated in Fig. 4 comprises essentially the limiting ampliiler 1 including the vacuum tube VT--I followed by the tuned transformer T which may be a voltage step-down transformer T having a primary winding P shunted by the tuning condenser Ct, and having a secondary winding S shunted by the resonant crystal circuit which includes the piezoelectric crystal I connected in series circuit relation with the two capacitors Cs and Ck. The capacitor Cs may be adjusted to operate the circuit at the resonant frequency of the series-connected crystal element I and capacitor Cs. The capacitor Ck, also being` part of the series capacitance in the resonant crystal circuit, may be used as a means for adjusting the frequency at oscillation to substantially exact resonance of the crystal circuit, and also it may be used to divide the circuit voltage and develop the output voltage eo across a relatively low impedance condenser Ck, to be used for feedback purposes as the input voltage to the control grid 22 of the limiting amplifier tube VT-I. As a result, the output voltage en, developed across the capacitor Ck has been iltered twice, once by the tuned primary winding P of the transformer T and again by the resonant crystal circuit itself, the result of such filtering being an output voltage eo which is relatively free from electrical harmonics. When the tuning condenser Ct is tuned for resonance, the effective capacitance across the crystal element I may be substantially the same as the value of capacitance in series with the crystal element I. By adjusting the ratio of capacitance of the first condenser Cs with respect to that of the second condenser Ck, the amplitude of oscillations from a given crystal element I may be varied. For example, if it is desired to hold the voltage appearing across the crystal element I to a low value, and still insure maximum sensitivity to starting oscillations of the crystal element I, the feedback voltage en may be made to correspond to the greater portion of the sum of the voltage en plus ee appearing across both of the series-connected condensers Ck and Cs. In the extreme limit, the threshold of the limiting voltage applied to the control grid 22 of the limiting tube VT--I may become eo plus ec, and it may also be equal to the voltage appearing across the crystal element I since the crystal circuit is resonant, thus making the voltage across the inductance characteristic of the crystal element I equal to the voltage across the series capacitance characteristic thereof. In this way, the voltage across the crystal element I may be controlled at any frequency between the resonant and antiresonant frequencies of the crystal body I, and at voltage levels down to a low value such as about one volt or less. At the same time, stability of operation and assurance of starting oscillations are of good quality.

The low level crystal oscillator circuit, as shown in Fig. 4, may be used for testing, or for aging of piezoelectric crystal elements I, or wherever an oscillator circuit having low distortion output may be desired. This type of oscillator will permit the oscillation of piezoelectric crystals l at very low voltage levels, the start of oscillation is certain even with a voltage of the order of one volt applied across the crystal element I, and this circuit will oscillate almost independently of the effective capacity across the crystal body I, the equivalent shunt capacitor Clc being varied between 10 and 100 micromicrofarads for example.

Although this invention has lbeen described and illustrated in relation to specific arrangements, it is to be understood that it is capable of application in other organizations and is therefore not to be limited to the particular embodiments disclosed.

What is claimed is:

1. A crystal-controlled oscillation generator comprising electronic amplier apparatus having input and output circuits, a tuned transformer having primary and secondary windings, said primary winding of said transformer being connected with said output circuit of said amplier apparatus, a resonant crystal circuit comprising a .piezoelectric crystal body, a rst capacitor connected in series circuit relation with said crystal body, and a second capacitor connected in series circuit relation with said first capacitor and said crystal body, said resonant crystal circuit being connected with said secondary winding of said transformer, and a feedback circuit connected from a point intermediate said rst and second capacitors of said resonant crystal circuit to said input circuit of said amplifier apparatus, said amplifier apparatus being provided with means for limiting and controlling the amplitude of said oscillation supplied to said output circuit.

2 A crystal-controlled oscillation generator comprising electronic amplifier apparatus having input and output circuits, a tuned transformer having primary and secondary windings, said primary winding of said transformer being connected with said output circuit of said amplifier apparatus, a condenser connected in shunt circuit relation with said primary winding for vtuning said transformer, said tuned transformer constituting means for adjusting the frequency of said oscillation, a resonant crystal circuit comprising a piezoelectric crystal body, a rst capacitor connected in series circuit relation with said crystal body, and a second capacitor connected in series circuit relation with said rst capacitor and saidicrystalbody, said resonant 'crystal circuit being connected across said secondary winding of said transformer, and a feedback circuit connected from a point intermediate said rst and secondcapacitors of said resonant crystal circuit to said input" circuit of said amplifier apparatus, said amplifier apparatus being provided with` means for limiting and controlling the amplitude of said oscillation supplied to said output circuit'.r

3. A crystal-controlled oscillation: generator comprisingY electronic amplifier apparatus havlng input and output circuits, a tuned transformer having primary and secondary windings, said primary Winding of said transformer being connected with said output circuit of said amplier apparatus, a condenser connected -in shunt circuit relation with said primary winding for tuning said transformer, said tuned transformer constituting means for adjusting the frequency of said oscillation, a resonant crystal circuit comprising a piezo.- electric .crystal body, a first capacitor connected in series circuit relation with said crystal body, and a second capacitor connected in series circuit relation: with said first capacitor and said crystal body, at least one of said capacitors constituting means. for adjusting the frequency of saidoscilla: tion, said resonant crystal circuit being connected across said secondary winding of said, transformer, and a feedback circuit connected from a point. intermediate said first and second capacitors'of. said resonant crystal circuit to said input circuit of said amplifier apparatus, said amplifier' apparatus being provided with means for limiting, and controlling the amplitude of said oscilla? tion supplied to said output circuit.

4.-.- A. crystal-controlled oscillationgenerator comprising electronic amplifier apparatus having input and output circuits, a tuned transformer having primary and secondary windings, said primary winding of said transformer being connectedv with said output circuit of said amplifier apparatus, a resonant crystal circuit comprising a' piezoelectric crystal body, a first capacitor connected in series circuit relation with said; crystal body, and a second capacitor connected in series circuitA relation with said first capacitor and said crystal body, said resonant crystal circuit being connectedwith said secondary winding of said transformer, and a feedback circuit connected from. a point intermediate said first and second capacitors of said resonant crystal circuit to said input circuit of said amplifier apparatus, said tuned transformer and at least one of said first and-second capacitors constituting means for adjusting the frequency of said oscillation between theresonance and antiresonance frequencies of saidcrystal body, said amplifier apparatus being providedwith means for limiting and controlling the-amplitude of said oscillation supplied to said output circuit.

5. A crystal-controlled oscillation generator comprising electronic amplifier apparatus having input andoutput circuits, a tuned voltage-step,- down. transformer having primary and secondary windings, said primary winding of said transformer being connected with said output circuit of said amplifier apparatus, a resonant crystal circuit comprising a piezoelectric crystal body, a first capacitor connected in series circuit relation with said crystal body, and a second capacitor connected in series circuit relation with said first capacitor and said crystal body, said resonant crystal circuit being connected with said secondary winding of said transformer, and a feedback-circuit connected from a point' intermediate:

I4 samarsizandcseccndrcapacitors-ofjsaid resonant crystal circuit: to said- `infinit :circuit .ofI 4said ampliilerapparatua said-V tuned transformerand at least one of saidr-rst and second capacitors constituting mean-s -for-adiusting; the frequencyI of said.- oscillatiom between the: resonance and antiresonance frequencies of' said crystal bodyl said ampiiierf apparatus 'being provided with means fon limiting andrcontrolling. the` amplitude of` said oscillation suppliedfto. said output circuit.

6.A A: crystal-controlled; oscillation generator comprising ra; .limiter amplifier; having `a voltage limitmgcharacteristic; aiseparatelinear amplifier comprisingsatileast ones vacuum tube having a substantially linear- -response characteristic, said linear. antplicnhavlng an input .circuit connected witihi the 'output circuit of said limiter amplifiery mtimed transformcrfhaving Vprimary and secondary; windings, said primary; winding of said transformer being-:connected withfthe output circuit .of saidlineary ampliflen aresonant crystal circuit c connected in shuutfl circuit relation with said secondary winding-ofsaid transformer, said resonant crystal circuit comprising apiezoelectric crystal body; a-;rst -capacitorconnectedin series circuit relation with. sald'crystal body, and a secondcapacitor. connected :in series circuit relation with said-:firstcapacitor armi.r said crystal body, a feedbaclccircuit connected from said resonant crystal jcircuit-toytheinput circuit of said limiter ampiider; and meansfincludingea rectier connected front-.saidresonant crystal circuit to said input circuit of said-linear amplifier for .controlling the-amplitude 'of oscillation in said, -resonant icrystal circuit,

'7. .A1 crystal-controlled` oscillationt generator comprising-.azy limiter amplifier having a voltage limiting characteristic, aseparate linear amplier comprislngat least .one lvacuum tube having a substantially linear v`response; characteristic, rsaid linearamplier having an input circuit connected withthe output circuit-of saidv limiter amplifier, a tuned transformer; having primary and secondary windings, saidprimary winding of said transformeribeing connected with the output circuit of; saidzlinear amplifier, avresonant crystal circuitconnected inshunt relation with said vsecondary winding'l ofA saidv transformer, said resonant crystal, circuitV comprising a piezoelectric crystal body, a rst capacitorv connected in series circuit relation with said crystal body, and asecond capacitor connected ingseries circuit relation with-.saidfirst capacitcrandsaid ,crystal body, a feedbackf circuit connected from said resonant crystalcircuittothe inputv circuit of.V said; limiter amplifienandmeans including a rectifier, and a direct current amplinerconnected from said resonant-.crystalcircuit-to said input circuit of said linearwamplierfor controlling theA amplitude of oscillation. in; said; resonant: crystal circuit, the time consta-nt of: salda last-mentioned amplitude controlmeans being slower than the time, constant of said limiteriamplier.

8:.A crystal-controlled, oscillation generator comprisinga limiteryamplier-*having a voltage amplitude; limiting: characteristic, aseparate Vlinear: ampl-.ier' comprisingy at least one vacuum tube having' a saibstantiallyL linearresponse characteristic, said linear; amplifier having an input circuitconnected-with the output circuit of said limiten amplifier; al tuned transformer'l having primary and;- secondary, windings-said primary windingof saidtransformer being-connected with the'. outputV circuitof; said.. linea-rf amplifier, a resonant crystallcircuitI connectedzin-zshuntrelation with said secondary winding of said transformer, said resonant crystal circuit comprising a piezoelectric crystal body, a first capacitor connected in series circuit relation with said crystal body, and a second capacitor connected in series circuit relation with said first capacitor and said crystal body, a feedback circuit connected from said resonant crystal circuit to the input circuit of said limiter amplifier, and means including a rectifier connected from said resonant crystal circuit to said input circuit of said linear amplifier for controlling the amplitude of oscillation in said resonant crystal circuit, the time constant of said last-mentioned amplitude control means being slower than the time constant of said limiter amplifier, said tuned transformer and at least one of said first and second capacitors constituting means for adjusting the frequency of said oscillation between the resonance and antiresonance frequencies of said crystal body.

9. A crystal-controlled oscillation generator comprising a limiter amplifier including an overloaded vacuum tube having a substantially at response characteristic, a separate linear amplifier comprising at least one vacuum tube having a substantially linear response characteristic, said linear amplifier having an input circuit connected with the output circuit of said limiter amplier, a tuned transformer having primary and secondary windings, said primary Winding of said transformer being connected with the output circuit of said linear amplifier, a resonant crystal circuit connected in shunt relation with said secondaiy winding of said transformer, said resonant crystal circuit comprising a piezoelectric crystal body, a first capacitor connected in series circuit relation with said crystal body, and a second capacitor connected in series circuit relation with said first capacitor and said crystal body, a feedback circuit connected from a point between said rst and second capacitors of said resonant crystal circuit to the input circuit of said limiter amplifier, and means including a rectifier connected from said resonant crystal circuit to said input circuit of said linear amplifier for controlling the amplitude of oscillation in said resonant crystal circuit, said tuned transformer and at least one of said first and second capacitors constituting means for adjusting the frequency of said oscillation between the resonance and antiresonance frequencies of said crystal body.

10. A crystal-controlled oscillation generator comprising a limiting type amplifier having an output circuit connected in tandem with and supplying oscillations to the input circuit of a separate linear type power amplifier, a transformer having primary and secondary windings, said primary winding being connected with the output circuit of said linear type amplifier, a series circuit comprising a piezoelectric crystal element, a plurality of series connected capacitors and said secondary winding of said transformer, said series circuit comprising the output circuit of said transformer, a feedback circuit connecting the output circuit of said series circuit with the input circuit of said limiting amplifier, and an automatic volume control circuit connected with and supplying rectified voltage from said output circuit of said transformer to said input circuit of said linear type amplier, said limiting type amplifier comprising a relatively fast-acting limiter of the amplitude of said oscillations and said automatic volume control circuit comprising a relatively slower acting limiter of the amplitude of said oscillations.

11. A crystal-controlled oscillation generator comprising an amplitude limiting amplifier, a separate linear amplifier connected in the output circuit of said limiting amplifier, a phase-shifting tuned transformer having a primary Winding connected with the output circuit of said linear amplifier, a series resonant circuit including a piezoelectric element and a capacitor and the secondary winding of said transformer, a feedback circuit connecting said series resonant circuit with the input circuit of said limiting amplifier, and automatic amplitude control means interconnecting the input and output circuits of said linear amplifier, the time constant of said limiting amplifier being faster than that of said automatic amplitude control means.

12. A crystal-controlled oscillation generator comprising a limiting non-linear type single-stage amplifier provided with means for limiting the amplitude of said oscillation, a tuned transformer having a primary winding and a secondary winding, said primary winding being connected across the output circuit of said limiting amplifier, a resonant crystal circuit shunted across said secv ondary winding of said transformer, said resonant crystal circuit comprising a piezoelectric crystal body connected in series circuit relation with a capacitor and an additional capacitor, said additional capacitor constituting means for adjusting the frequency of said oscillation to resonance in said crystal circuit and for developing an output circuit voltage across its impedance, and a feedback circuit connecting said output circuit voltage with the input circuit of said limiting amplifier.

13. A crystal-controlled oscillation generator comprising a limiting non-linear type amplifier, including an overloaded vacuum tube constituting means for limiting the amplitude of said oscillation, a tuned transformer having a primary winding and a secondary winding, said primary winding being connected with the output circuit of said limiting amplifier, a resonant crystal circuit shunted across said secondary winding of said transformer, said resonant crystal circuit comprising a piezoelectric crystal body connected in series circuit relation with a capacitor and an additional capacitor, said additional capacitor constituting means for adjusting the frequency of said oscillation to resonance in said crystal circuit and for developing an output circuit voltage across its impedance, and a feedback circuit connecting said output circuit voltage with the input circuit of said limiting amplifier, said transformer being a step-down transformer for supplying a reduced voltage to said resonant crystal circuit.

14. A crystal-controlled oscillation generator comprising a single-stage, voltage limiting nonlinear type amplifier, a tuned transformer having a primary winding and a secondary winding, said primary winding being connected across the output circuit of said limiting amplifier, a resonant crystal circuit shunted across said secondary winding of said transformer, said resonant crystal circuit comprising a piezoelectric crystal body connected in series circuit relation with a capacitor and an additional capacitor, said additional capacitor constituting means for adjusting the frequency of said oscillation to resonance in said crystal circuit and for developing an output circuit voltage across its impedance, and a feedback circuit connecting said output circuit voltage with the input circuit of said limiting amplifier, said transformer being a step-down transformer for supplying a reduced voltage to said resonant crystal circuit, and the ratio of the capacitances of said capacitor with respect to said additional capacitor being a value corresponding to the desired amplitude of oscillations of said crystal body.

15. A crystal-controlled oscillation generator comprising means for amplifying and limiting the amplitude of said oscillation, said means comprising a single vacuum tube having a substantially i'lat response overloaded characteristic, a tuned step-down transformer comprising primary and secondary windings and a tuning condenser disposed in shunt circuit relation with said primary winding, said primary winding being connected in shunt with the output of said limiting ampliiier tube, a resonant crystal circuit connected in shunt with said secondary winding of said transformer, said resonant Vcrystal circuit com- 18 prising a piezoelectric crystal element, a first ca.- pacitor connected in series circuit relation with said crystal element, and a second capacitor connected in series circuit relation with said first capacitor and said crystal element, and a feedback circuit connected from a point intermediate said first and second capacitors in said resonant crystal circuit to the input circuit of said limiting amplier vacuum tube, said tuned transformer and one of said capacitors constituting means for adjusting the frequency of said oscillation between the resonance and antiresonance frequencies of said crystal element, and the ratio of the capacitances of said rst and second capacitors being a Value corresponding to the amplitude of oscillation of said crystal element.

CHARLES W. HARRISON. 

